Mine roof supports are known in a variety of designs. In one form, such a support comprises a pair of floor skids or sills bearing hydraulic props which carry a roof engageable structure and a goaf shield is pivotably supported on the roof engageable structure. To shift the support an hydraulic ram and a floor beam are disposed one above the other between the floor sills. The floor beam is coupled to a scraper-chain conveyor and the shifting ram is operated to advance the support up to the conveyor or to thrust the conveyor forwardly relative to the mineral face. In order to assist the shifting ram, especially in workings where the floor skids are prone to become embedded into the floor, it is known to use a lifting mechanism to raise the skids partially at least at their front end regions when the shifting ram is operated.
Published patent specifications U.S. Pat. No. 4,600,340 and GB-2237837 both described supports of the aforementioned kind. In GB-2237837 the lifting mechanism takes the form of an upstanding piston and cylinder unit which is connected to a bridge member which interconnects the floor skids and extends over the space between the floor skids. By extending the unit the unit engages on the floor beam to press this against the floor to partially raise the skids. It is generally desirable to provide a structure which is able to be adapted to the nature of the prevailing conditions without necessitating an hydraulic lifting unit of excessive length. In GB-2237837 provision is made to secure the lifting unit to the bridge member in alternative positions. Even when the unit is fully retracted the unit can still contact the floor beam if the floor skids sink into the floor and damage and distortion can occur.
A general object of the present invention is to provide an improved roof support.